Negative thermal expansion materials are unique in that they expand upon cooling and contract upon heating. By combining negative thermal expansion materials with other common materials, such as ceramics, compositions can be formed which resist expansion on heating and contraction upon cooling. Negative thermal expansion materials also are useful for adjusting the thermal expansion of a composition to match that of another material which it contacts. Particularly useful are NTE materials which expand or contract isotropically, i.e, to substantially the same extent in all dimensions.
A number of U.S. patents concern negative thermal expansion materials and methods for making such materials, including Arthur Sleight's U.S. Pat. Nos. 5,322,559 and 5,514,360, which are incorporated herein by reference. Sleight's '360 patent describes a method for making zirconium and hafnium tungstates. The method involves heating mixtures comprising appropriate amounts of starting reagents. But, the temperatures required to produce single-phase compounds have heretofore been higher than 1,100.degree. C. Sleight's '360 patent states that "the production of single-phase compounds generally has required heating temperatures to be at least as high as about 1165.degree. C. [and] typically should be from about 1165.degree. C. to about 1250.degree. C. . . ." Sleight's '360 patent, column 3, lines 17-19. Example 5 of Sleight's '360 supports this conclusion. The material produced according to Example 5 of Sleight's '360 patent includes both crystalline ZrW.sub.2 O.sub.8 and WO.sub.3.
Several literature reports also discuss the production of zirconium and hafnium compounds. See, for example, (1) Clearfield and Blessing's "The preparation of a Crystalline Basic Zirconium Tungstate," J. Inorg. Nucl. Chem., 36:1174-1176 (1974); and (2) S. Palitsyna et al.'s "Synthesis and Some Properties of Basic Crystalline Hafnium," Bulletin of the Academy of Sciences, U.S.S.R., Division of Chemical Sciences, 26:611-613 (1977). Clearfield et al. teach a method for synthesizing ZrW.sub.2 O.sub.7 (OH).sub.2 (H.sub.2 O).sub.2 by combining zirconium oxychloride octahydrate (ZrOCl.sub.2.8H.sub.2 O) with sodium tungstate (Na.sub.2 WO.sub.4.2H.sub.2 O), followed by heating the precipitate and mother liquor formed by the combination. The solution is acidified and refluxed for "several days." Clearfield et al., supra, p. 1175. The solid is then collected and washed with hydrochloric acid to remove sodium ion. Id.
Palitsyna et al. describe the synthesis of a hafnium tungstate by combining hafnium oxychloride octahydrate (HfOCl.sub.2.8H.sub.2 O) with sodium tungstate dihydrate (Na.sub.2 WO.sub.4.2H.sub.2 O). The hydrate formed by this reaction and subsequent workup is then heated to temperatures greater than 500.degree. C. (presumably celsius; the publication does not say) to form the crystalline hafnium tungstate.
Several problems have been identified with methods developed prior to the present invention for making zirconium and hafnium tungstates. First, such methods produce substantially single-phase compounds only at elevated temperatures, and generally only after long heating periods, which are significant drawbacks to developing an efficient, commercially viable process. Second, previous methods limited the number of different NTE compounds that could be made because they allowed only for the substitution of hafnium for zirconium; no other elemental substitutions apparently have been possible using prior synthetic methodologies. And, prior methods ostensibly designed to make zirconium and hafnium tungstates do not reproducibly produce such compounds. In fact, commercial preparations sold as zirconium tungstate have been found to include little or no zirconium tungstate.
Based on the above, it is apparent that there is a need for a new method for reproducibly forming known negative thermal expansion materials as substantially single-phase compounds at substantially reduced temperatures relative to known methods. There also is a need for a method which allows for the synthesis of novel negative thermal expansion materials.